1. Field of Invention
This invention relates to a device and method for realizing the function of a nonlinear comb filter and, more particularly, to a device and method for realizing the function of a median comb filter for filtering harmonic interference from a transient or impulsive signal o interest.
2. Description of Related Art
Interference (i.e., noise) has typically been removed from signals by using linear filters, but linear filters tend to introduce artifacts or noise when the signal of interest is either a transient signal or an impulsive signal. For example, a low pass filter used on a speech signal will filter out high frequency noise along with important high frequency speech components. The result is a distorted speech signal.
A particularly troublesome noise problem is created by interference from power line systems that corrupt impulsive signals (as oppossed to periodic signals). Power line systems typically transmit energy at some fundamental frequency (e.g. 60 Hz). The interference signal created by a power line system typically consists of harmonic frequencies of this fundamental frequency. These harmonic frequencies are multiples of the fundamental frequency of the trasmitted signal (e.g., 60 Hz, 120 Hz, 180 Hz, 240 Hz, etc.). Such interference is known as harmonic interference.
The conventional way to reject or reduce harmonic interference is to use some form of linear filter such as a notch filter or comb filter. As the bandwidth of the linear filter narrows, the non-negligible portion of the impulse response grows longer in time. This fundamental property applies to all linear filters (i.e., bandpass, notch, highpass, lowpass, etc.). Consequently, any linear filter will have a non-negligible impulse response resulting in echo-like artifacts when a transient (i.e., impulsive) signal is processed. Another way of stating this is that narrowband linear filters will "ring" when hit by an impulsive signal.
The "ringing" (or echos) produced by linear notch filters and linear comb filters appear as replicas of the impulsive signal but with decreasing amplitude at intervals equal to the fundamental period of the interference signal being filtered out (i.e., at every 0.01667 sec. for a signal with a fundamental frequency of 60 Hz).
Nonlinear devices (such as median devices) are useful for preserving monotonic trends (i.e., non-impulse trends) and steps within a signal However, nonlinear functions (such as the median) by themselves do not remove harmonic interference.
The median filter was introduced by J. W. Tukey in Exploratory Data Analysis, Addison Wesly, 1971. The median filter consists of a tapped delay line connected to a median device. The basic idea is to slide a fixed length sampling window across the input data. At each sample point, a median function is performed on the values that are currently within the sampling window. The median filter is useful for removing noise spikes within monotonic (non-impulsive) signals while leaving the monolithic steps unaffected.
For example, a signal stream may consist of the values - 3 4 10 3 5 15 16 14 12 -. The dash (i.e., "-") represents an unknown value. Using a delay line with three taps, the following samples would be presented to the three-input median device: (- - 3), (- 3 4), (3 4 10), (4 10 3), (10 3 5), (3 5 15), (5 15 16), (15 16 14), (16 14 12), (14 12 -), and (12 - -). The median filter would produce the following output signal: - - 4 4 5 5 15 15 14 - - . Notice that the impulsive signal (10) was removed while the step (5 to 15) was preserved.
In "A Separable Median Filter for Image Noise Smoothing", a published article by Patrenahalli M. Narendra in IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. PAMI-3, No. 1 Jan. 1981, pp. 20-29 it was pointed out that a median filter is more effective than a linear filter in smoothing out noise in image signals
In "Adaptive Cancellation of Periodic, Synchronously Sampled Interference", a published article by S. J. Elliott and P. Darlington in IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-33, No. 3, Jun. 1985, pp. 715-717 harmonic interference is synchronously sampled and linearly summed to cancel noise using a linear comb filter The samples were required to be spaced at exact multiples of the fundamental frequency of the interference signal
In "Adaptive Harmonic Noise Cancellation with an Application to Distributed Power Line Communications", a published article by Jin-Der Wang and H. Joel Trussell in IEEE Transactions on Communications, Vol. 36, No. 7, Jul. 1988, pp. 875-883 it was indicated that serious problems exist in power line distribution systems due to drifting power line frequency and harmonic bandwidth. It was mentioned that systems that use fixed filters cannot take variations in harmonic noise into account but that adaptive filters could. Adaptive filters have previously been used to cancel noise but this paper showed that they could also be used to cancel harmonic noise. Adaptive filters have two inputs. One input is used for the noise-corrupted signal The other input is used for the reference signal The reference signal is made up of samples of the corrupted signal. The samples are taken at integral multiples of the power line fundamental frequency. Adaptive algorithms adjust the taps of the filter in order to predict the noise in the corrupted signal. The predicted noise is then subtracted from the corrupted signal in order to cancel the noise. Harmonic noise tends to occur at multiples of the power line fundamental frequency (e.g., 60 Hz). The frequency of harmonic noise also varies as the power line fundamental frequency varies This paper proposed a solution to this problem by using a linear comb filter embodied in a finite impulse response (FIR) filter as the adaptive filter.
In "On Computation of the Running Median", a published article by Jaakko T. Astola and T. George Campbell in IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. 37, No. 4, Apr. 1989, pp. 572-574 it was mentioned that digital filters could be used to process signals in the time or frequency domain. Digital filtering has been accomplished by linear filters which tend to distort abrupt changes in signals. Linear filters are also not able to remove impulsive noise totally. Median filters have been used to get around these problems. A continuously sampled or running median filter was proposed to get an unbiased estimate of a signal of interest.
In U.S. Pat. No. 4,928,258, entitled "Recursive Median Filtering", a device was disclosed that performs the function of a median filter in a new way (i.e., recursively). This invention is solely a new way of doing the nonlinear median function. The present invention relates to using a median filter within a comb filter structure in order to realize a nonlinear comb filter.
In U.S. Pat. No. 4,724,395, entitled "Median Filter for Reconstructing Missing Color Samples", a device was described that utilizes a median filter to process image signals. This invention does not describe the use of a median filter within a comb filter structure as the present invention does.